The invention relates generally to heat recovery systems, and more specifically, to heat recovery systems that employ two or more organic Rankine cycle (ORC) units disposed in series.
Heat recovery systems are frequently employed to recover low-grade heat, such as heat with a temperature below approximately 500 to 1000° C., from industrial and commercial processes and operations. For example, heat recovery systems may be employed to recover geothermal energy, heat from hot exhaust gases produced by gas turbines or by reciprocating engines, heat from cooling water after it has cooled a process, or heat from flue gases used in industrial processes, among others. Heat recovery systems that implement an organic Rankine cycle by circulating an organic working fluid may be particularly efficient at recovering low-grade heat due to the relatively low phase change enthalpies of organic working fluids.
A typical ORC unit circulates an organic working fluid through a closed loop to convert heat into work. The working fluid is heated in an evaporator where the working fluid is evaporated to produce a vapor that is expanded across a turbine to turn the turbine shaft. The rotation of the turbine shaft drives a load, such as a generator, which produces electrical power. The expanded working fluid is then directed through a condenser where the vapor is condensed into a liquid. The liquid working fluid is then pressurized by a pump that returns the working fluid to the evaporator. Due to turbine load restrictions, design modularity desires, or other constraints, it may be desirable to employ multiple ORC units within a process to generate a desired amount of turbine output power.